1. Field of Invention
The present invention relates to solid photochromic polymers including coatings, free-standing films, and solid articles that exhibit variable transmission of light upon exposure to ultraviolet radiation. The photochromic polymers are useful for controlling the transmission of light in building and automobile windows, sunroofs, ophthalmic plastic lenses, and any surface that would benefit from variable transparency in direct sunlight.
2. Background of the Art
Polymer articles that have organic photochromic dye(s) applied or incorporated therein are characterized in that upon exposure to electromagnetic radiation, e.g., solar radiation, they exhibit a reversible change in color and light transmission. Once the exposure to the activating radiation has been discontinued, the composition returns to its original color, or colorless state. Photochromic plastic materials, most notably, such as compositions suitable for variable transparency in direct sunlight, have applications in many architecture, building, and automotive glazing applications, as well as for ophthalmic lenses and other solid objects. A general and informative review of photochromic organic materials is presented in “Photochromism, Molecules and Systems” by H. Dürr and H. Bouas-Laurent, eds., Elsevier, Amsterdam, (2003).
A wide variety of polymer materials have been investigated as host materials for photochromic dyes systems. Diallyldigylcolcarbonates (e.g. CR-39, from PPG Industries) and related polyol(allylcarbonate) systems are described in U.S. Pat. No. 5,246,630, and references cited therein for use with photochromic dyes. The systems require either a photochromic dye which is resistant to the effects of peroxy type initiator or a secondary processing step in which the photochromic dye is “imbibed” or carried into the polymer by solvent/thermal transfer. U.S. Pat. No. 4,994,208, McBain, et al., describes compositions that comprise a mixture of minor amounts of a acrylate capped polyurethane and a major amount of the polyol(allylcarbonate) composition that improves the equilibrium response of the photochromic dyes compared to photochromic articles prepared from homopolymers such as diethyleneglycol bis(allylcarbonate). The systems tend to undergo yellowing with heat ageing and thermal transfer. However, U.S. Pat. No. 5,084,529 and the earlier cited '630 describe the use of small amounts of pyrocarbonate and triphenyl phosphite, respectively, to circumvent the heat related yellowing problem.
Photochromic polyurethanes are described in EU 0294056, Ormsby, and later in U.S. Pat. No. 6,107,395, Rosthauser, et al., The significant benefit of the polyurethane polymer matrix is that photochromic dyes are typically stable to the cure or hardening process. The '395 patent also represents the polyurethane-photochromic dye system to have superior coloring and fading rates. U.S. Pat. No. 5,498,686, Effer, et al., describe a polythiourethane matrix derived from polymerization of diisocyanate with a polymercapto compound, that has improved photostability and thus improved cycle and service life over the conventional materials based on diethylene glycol bisallyl carbonate systems.
Many other polymers have been used to make photochromic articles, but for most applications that require sunlight and thermal stability, they have been found to be lacking. The critical response requirements of a photochromic polymer system include: photo-activation (coloring: quantum yield and intensity), fading (rate of bleaching in absence of light), and thermal and photostability in terms of life cycle. There are also other performance issues such as transparency and dispersion, impact resistance and scratch resistance that are highly dependent upon the specific polymer matrix chosen to hold the photochromic dye.
Surprisingly, the inventor has found that addition of a small amount of a specific class of material to a photochromic polymer system can significantly improve the photo-activation and fading response of a given photochromic system. That class of material is commonly referred to as an electron acceptor. However, the invention is not defined or limited by what the mechanism of action may or may not be.